gothicsurf

I have a 94 pickup with the engine light on; when connecting the diagnostic jumper (TE1 to E1) the blink code reads an O2 malfunction. The engine light would go on occasionally after climbing a grade and then eventually go off again, but now it persists. The truck does feel to be running either too rich or too lean so I would like to replace the sensor but do not know which: the upstream or the downstream, or both? I understand they usually are replaced every 60-80,000 anyway?

scope103

Are you sure? There are a least two codes that have to do with the O2 sensor circuit. And like every other code, they do not tell you what part to replace.

Let me guess that you got code 21, http://personal.utulsa.edu/~nathan-b...85diagnosi.pdf which is an open or short in the upstream O2 sensor heater circuit. While it is likely that it is the heater in the sensor itself that is bad (meaning you need to replace the sensor), and replacing an O2 sensor after 100k miles is probably a good idea even with no problems, don't be too surprised if that doesn't clear the code. Instead, you could just test the sensor first to be sure. http://personal.utulsa.edu/~nathan-b...01heatedox.pdf.

Note that an open heater circuit only makes a difference at idle; if you get a CEL on a grade the sensor is warm enough on its own. So if you're running rich or lean that's probably unrelated to code 21.

Since you didn't tell us the code, I suppose you could have gotten code 27, which is the heater circuit to the downstream sensor. You also didn't tell us where you're from; only the finest trucks (California) have a second O2 sensor.

You can just start throwing parts at your problem; you'll have lots of company. But as you can probably guess, you'll be spending money for a long time with no results.

Good luck!

gothicsurf

true, a California truck and the engine code is 21. If California is one of the only places to have downstream O2 sensors, then it would seem to follow that the CEL is referring to the upstream sensor. Unfortunately I don't have an ohmmeter or a voltage meter with a fast enough response to do the tests. I've had the truck from 133 to 162 thousand, so it may very well be due. Anyone have ideas of where I can find a reliable sensor, thinking OEM since the Bosch has had some spotty reviews?

scope103

A little knowledge is a dangerous thing.

You only need the fast-response voltmeter to test the feedback voltage (VF1). A good test to do, because it evaluates whether the sensor is correctly measuring O2. A fast-response meter is pretty easy to get. http://www.radioshack.com/product/in...uctId=2103176#

But it doesn't have anything to do with your problem. Your code is based on an open/short in the heater circuit, and the test for that is just measuring the resistance between two terminals on the O2 sensor. Any $8 meter will do that just fine. (or splurge; get the Radio Shack meter and you'll be set for a very long time. You've read enough of the FSM to know that a multimeter is essential for any diagnosis).

Or, drop $75 on a new O2 Sensor. If you're right, you've saved $40, at least until the next problem in your truck comes up. But if it doesn't make the code go away, you're still in the dark. Your choice.

[I bought the cheapest O2 sensor RockAuto sold, and it's worked just fine. Stay away from the universal fit sensors; you can't use the OEM connector, so you end up having to cobble together something in the worst place possible.]